In many aerosol spray applications, it is desirable to deliver a spray of small particles (1-200 microns in diameter) having generally uniform diameters. Uniformly small particles, also referred to as droplets, when applied to a surface, can coalesce into thin surface coatings having uniform thickness. Such consistently and predictably thin coatings dry more rapidly and evenly than coatings formed from aerosol systems that deliver droplets of variable sizes. It is also believed that such variably thick spray coatings, due to their associated uneven drying times, may not bond to surfaces as strongly as coatings formed from uniformly thin droplets. This is particularly undesirable in applications where permanent and uniform coating/surface bonding is essential, such as with spray paints and adhesives. Hence, there are several techniques which have been used to reduce particle size.
In conventional aerosol spray systems, propellants have been applied to reduce aerosol particle size. In addition to providing the pressure required to force the aerosol out of the container when the actuator is depressed, the propellant plays an essential role in fluid particle formation and the overall spray characteristics of the aerosol. When the propellant/fluid mixture is discharged from such aerosol dispensing systems, fluid particles are initially formed as a result of the vaporization of the propellant and the kinetic energy imparted by the propellant to the liquid. Particle size continues to reduce as the particle travels farther from the dispenser orifice as a result of further propellant vaporization and release of kinetic energy.
Thus, choice of propellant is a critical consideration in aerosol droplet formation. In general, the vapor pressure and concentration of the propellant are the variables that most directly affect aerosol droplet size. As the concentration or vapor pressure of the propellant increases, droplet size typically decreases.
Droplet size also depends on the viscosity of the fluid. Many high viscosity polymers, such as those used in adhesives and paints, are incapable of being sprayed as small droplets using conventional aerosol dispensing systems, even if propellants having high vapor pressures and/or concentrations are used. To make such high viscosity materials sprayable, a solvent that is compatible with both the fluid and propellant of the aerosol mixture must be added. To obtain sprayable mixtures of such high viscosity materials, solvent solutions having 20% or less by weight solids content are often required.
The actuator or nozzle design also influences aerosol droplet size. Orifice size and taper can be manipulated to tailor droplet size, as well as alter the aerosol spray pattern. Designs that atomize the fluid stream by diverting the propellant within the actuator (so called "mechanical break-up actuators") have also been developed. Such designs form smaller droplets by first inducing a swirling motion of the fluid within the actuator. When the swirling liquid exits the actuator orifice, atomization of the aerosol is enhanced over conventional systems due to the tangential motion of the swirling aerosol formulation.
All of these approaches in reducing aerosol droplet size, however, have associated drawbacks in producing uniformly small droplets. In conventional contained aerosol spray systems, the actual propellant concentration of the aerosol as it leaves the dispensing system is in continuous flux, even though an average overall propellant/fluid ratio exists. This variability is a result of several factors. In particular, the unavoidable uneven mixing of aerosol components in such contained systems results in lack of constant propellant concentration, resulting in a variability of aerosol droplet sizes.
Fluctuation in vapor pressure is also inevitably present in such systems. The turbulent fluid flow that is used to propel the aerosol from the dispensing system is variable by definition, relying on a continuously changing oscillatory force to drive the aerosol from the canister. It is also well known that the pressure in such systems decreases as the container empties. Both of these factors further contribute to variability of droplet sizes using these dispensing systems.
Although effective in transforming polymeric materials into sprayable mixtures, in many situations and locations the use of solvents is undesirable and/or not permitted. In the coatings industries, particularly in the development of adhesive products, efforts have been undertaken to remove solvents from formulations.
Water-based sprayable polymeric materials have been developed as alternatives to solvent borne aerosol formulations, but must often be formulated at lower solids levels and viscosities to be effectively atomized using conventional aerosol systems. Several drawbacks are associated with these reductions in solids content and viscosity. Lower solids content results in less deliverable material in a given canister or reservoir volume, translating into greater inconvenience and expense in using such materials. Lower viscosity polymers, by definition, typically also possess inferior physical and mechanical properties when compared to solvent-based, higher viscosity polymers. Thus, solvent elimination can also lead to performance compromises and concessions in sprayable polymeric formulations.
Furthermore, conversion to water-based formulations has resulted in a greater tendency for solidified polymeric and other materials to accumulate within the conventional aerosol system actuator and orifice. Solids accumulation and clogging had not been a significant problem in solvent-based aerosol systems because solids buildup was prevented and/or quickly dissolved by the solvent's presence in the system. Such solids buildup within the actuator associated with the use of water-based formulations, however, diminish the effectiveness of mechanical break-up systems in atomizing aerosols by clogging or altering the internal actuator channels that provide enhanced droplet formation.
What is desired is a system for atomizing liquids into a narrow distribution of particle size without the use of propellants or solvents. Additionally, it is desirable to have a system which atomizes high viscosity liquids or liquids with greater than 17% solids. It is also desirable to have a modular system with replaceable liquid and atomizing agent modules.